The present invention generally relates to metal-oxide-semiconductor field-effect transistors (MOSFET), and more specifically, to providing relatively smaller energy gaps for the source and relatively larger energy gaps for the drain of narrow band gap non-planar MOSFETs.
The MOSFET is a transistor used for amplifying or switching electronic signals. The MOSFET has source, drain, and metal oxide gate junctions or electrodes. As the push to increase the density of transistor devices on a chip continues, non-planar MOSFET structures have been developed, including for example fin-type FETs (FinFETs) and gate-all-around (GAA) nanowire FET structures. In FinFETs, the transistor channel is formed as a vertical fin with the gate wrapped over the fin between the source and drain regions such that the gate is on three sides of the channel. As fin widths decrease, however, channel width variations can cause variability and mobility loss in FinFETs. A GAA FET addresses this variability by placing the gate on all four sides of the channel. A GAA nanowire, for example, is essentially a silicon nanowire with a gate around the nanowire circumference. A GAA nanosheet is a three-dimensional silicon nanosheet with a gate formed around all four sides as well as the surface perpendicular to all four sides. The formation of a replacement GAA nanosheet transistor, like the formation of a replacement gate FinFET, generally involves the formation of a dummy gate used for source and drain formation followed by removal of the dummy gate and replacement with a metal gate.